Comparator circuit



Dec.'23, 1947. w. R. MERCER COMPRATOR CIRCUIT Filed May 4, 1944 PatentedDec. 23, 1947 COMPARATOR CIRCUIT William R. Mercer, Hightstown, N. J.,assignor to Radio Corporation of America, a corporation of DelawareApplication May 4, 1944, Serial No. 534,114

This invention relates to radio locator apparatus, and particularly toamplitude comparator circuits for target seeking devices and the like,wherein signals are directively radiated toward a selected target,picked up after reection thereby, and the received signals employed tocontrol the energization of a steering motor or similar device.

In the practice of the present invention, it is proposed to radiatesignalsalternately in two overlapping directive patterns, and comparethe amplitude of signal reilected with one radiation pattern with thatof the signal reected with the other radiation pattern. If the twoamplitudes are equal, the reecting target lies on the equi-signal lineof intersection of the alternate radiation lobes. If the signals areunequal, the

`target lies in the lobe from which the greater signal is reflected. Inorder to provide response to the inequality of the reected signals, acircuit capable of comparing the amplitudes of the signals receivedduring successive periods of radiation in alternate directive patternsis required. In copending application Serial No. 527,292 led by R, C.Sanders, Jr. on March 20, 1944, and entitled Comparator circuits forradio locators, certain circuits for this purpose are described andclaimed.

The principal object of the present invention is to provide an improvedcircuit of the general type disclosed in said copending application.Another object is to provide improved methods of and means forcontrolling an electric motor .or the like as a function of the relativeamplitudes of two alternately occurring signals. A further object is toprovide an improved comparator circuit requiring a minimum of electrondischarge tubes and other circuit elements yet capable of providingaccurate comparison of the amplitudes of alternate signals.

The invention will be described with reference to the accompanyingdrawing, of which'Figure 1 is a schematic diagram of a radio targetseeking system adapted for controlling the direction of flight of anaircraft in response to reiiected signals, and Figure 2 is a diagramillustrating a modification of the antenna arrangement of Figure 1.

The drawing shows one embodiment of the invention applied to a frequencymodulated radio locator system comprising a radio transmitter I which iscyclically frequency modulated by a frequency modulator 3, such as avariable capacitor unit, in response toa modulated signal 5 appliedthereto from a wave shaping circuit 1.

6 Claims. (Cl. Z50-1.72)

2 In the kexample illustrated, the modulating signal 5 is of triangularwave shape.

The triangular wave 5 is produced by periodically` opening and closing aswitch 9 in series with a battery II and a voltage divider I3. Thisgenerates a voltage of square wave form I5 thatV is applied to the waveshaping circuit 1 to obtain the voltage of triangular wave form 5. In

this case,` the wave shaping circuit 1 may be merely an integratingcircuit. The switch 9 is operated by a cam I1 driven by a motor I9.

The output of. the transmitter I is radiated alternately from a pair ofdirective antennas 2i and 23. This is accomplished by means of a switchcomprising switch arms 25 and 21 connected together mechanically foroperation by means of a cam 29. The cam 29 is connected to the motor I9.After reflection from a target the frequency Ymodulated signals arereceived by a directive antenna 3| and applied to a -beat frequencydetector 33. The frequency modulated signals are also supplied to thedetector 33 over a line 35 directly from the transmitter I, whereby thefrequency of the beat output of the detector 33 is proportional to thedistance to the target. This distance may be indicated by a meter 31connected to the output circuit of a frequency counter 39, which isconnected to the output of the detector 33 through a limiter 4I. Forobtaining directional information, output signal from the detector 33 isapplied to an audio amplifier 43 connected to an amplitude comparatorcircuit comprising a rectifier 45, a storage capacitor 41 andsynchronous switch 49, and a relay amplier 5I. The output circuit of theamplifier 5I includes a pair of relays 53 and 55, each provided withsingle pole double throw contact arrangements for connecting a battery51 to a motor 59.

`The rectier 45 comprises a grid leak type detector circuit including anelectron discharge tube 6I provided with a grid capacitor 63, grid leak65, grid current limiting resistor 61, and a load resistor 69. In theillustrated circuit, the tube 6I is of the pentode type, with screenpotential provided by a voltage divider including resistors 1I and 13connected across the anode supply voltage.

The anode of the tube 6| is connected through a resistor 15 to oneterminal of the capacitor 41.` The other terminal of the capacitor 41 isconnected to the moving contact 11 of the switch 49, which includes twofixed contacts 19 and 8I. The contact 11 is operated by means of a cam83 connected to the motor I9, in synchronism with ner o3, but inopposite phase.

'that the antenna 23 isY connected to' the transthe switches 25 and 21.The Contact 8| is connected to ground. The -contact 19 is connected tothe amplifier 5I through a low pass filter circuit comprising capacitors85 and 81 and a resistor 89. A resistor 9| is connected across thecapacitor 85, an-d together with a resistor 89 constitutes a grid leakfor the amplifier 5I.

The relays 53 and 55 are adjusted so that the drop out current of relay53 exceeds the pick up current of relay 55 by a small amount. Forexample, the relay 53 may be adjusted to pick up with a current of 4.6milliamperes and drop out Y at 4 milliamperes, while the relay 55 isadjusted to pick up at 2.6 milliamperes and 4drop out at 2 milliamperes.Thus, when the output current of the amplifier 5I is less than 2milliamperes, both relay armatures are in their lower positions asindicated in solid lines in the drawing. When y the current is between2.6 miiliamperes and 4 milliamperes, the armature of the relay 55 willbe in its upper position as indicated in dash line, while the armatureof the relay 53 will remain in its lower position. When the current isgreater than 4.6 milliamp'eres', the armatures of both relays will be intheir upper (dash line) positionsl` Thus, when the output current of theamplifier 5I has a value within thev range of 2.6 to 4 mi-lliamperes,the armature of thek motor 59 is disconnected from the battery 51 andshortcircuited. When the `output current is above this range, the motor5g is connected to run in one direction, and when the current is below 2milliamperes", the motor is connected to run in the opposite direction.The motor 5S may be meehanically connected to the rudder or othersteering mechanism (not shown) of a mobile craft carrying the equipment.v

In' the operation oi the above-described system, the motor lil drivesthe ca'rn i'.' to' modulate the transmitter I, and the cams' 29' and83,k to switch the antennas and the comparator circuit. The cam i1includes two lobes.. while the cams 29 and 83 each include one lobe. The'purpose of this arrangement is to provide a complete frequencymodulation cycle during each period of Ytrans'- mission on each of theantennas 2l and 23. The amplitude of the beat output oithev detector 33depends upon the strength of the refiecte'd sign-al. Hence, if thereflecting target is on the equisi'gnal line ci the directivel patternsof the antennas 2l and the heat output will not vary inamplitude withthe operation oi-- the switches 2'5, 2l andy dit. However, if the targetis to the left of the equi-signal line, a' stronger signal will bereceived `during the time that the antenna 2| is being used fortransmissionvand weaker diuring the time that the antennay 23 is used,-causing the beat output of the detector' 33 t'o" have a substantiallysquare wave amplitude envelope. The phase of this envelope withrespecttol the switching cycle is constant.` Similarly if the target is to the'right of the equi-signal' line', a similar square wave envelope will beproduced but of opposite phase.

The tube 5l is biased by its own grid current.

Thus, when the amplitude of the output of the amplifier i3 is low, theaverage anode current of the tube E! is high, and vice versa. Inasrnuchas the anode currentthrough'vthe-tube t?!` necessarily unidirectional,the output voltage appearing across the loa-d resistor es isunidirectional, following the envelope of the output ofi the ampli-During time mitter I', the capacitor i1 is connected to' groiindi tion,for example clockwise.

through switch Q9. The capacitor 41 is thus charged to a voltagedepending upon the strength of the reflected signal, in the polarityindicated by the signs -land The value of the resistor 15 is selectedwith respect to that of the capacitor 41 to provide charging to a Valuebetween the peak value and the `average value of the Voltage across theIload 'resistor 85. the antenna 2 I is connected to the transmitter I,the capacitor A1 is connected to the input circuit of the amplifier 5l.The Voltage drop at `this time across the load resistor 69 depends uponthe strength of the signal reflected from the target in response totransmission of the antenna 2l The Voltage at the upper end of theresistor 69 is positive with respect to ground. Thus the voltageapplied.- to the input circuit of the amplifier 5| isrthe differencebetweenv that at the upper end of the resisto-r 69 and that to which thecapacitor 41 is charged, If the reected signal is larger from theantenna 23 than the antenna' ZI, the average voltage applied totheamplifier 5l will be positive, since the capacitor [Il will be chargedtoa lower voltage than the voltage existing at the anode of t-he tube Siwhile the antenna 2l is connected.- Similarly, the voltage at the inputof the amplifier 5I will be negative it the reflected signal from the-antenna 23 is larger than that from the antenna 2l. When the reflectedsignals are equal', the input voltage to the' amplifier 5i is Zero. Thearnpliier 5! is biased by means of a cathode resistor 93 to" provide ananode current 0i approximately three milliampere's, when the inputsignal is zero. Thus, when the target is off to the left of theequi-signal lin'e', the anode current is decreased and the armatures ofthe relays 53 and 55 fall to their lower positions connecting the motor5@ for rotation in one ldirec- When the target is on to the right or theequi-signal line, the motor 59v is similarly energized to rotatecounter-clockw1se.

In the described embodiment o'f the invention, the transmitting antennasare alternately energized to'` providev overlapping radiation lobes, andthe receiving antenna is directed along the equisign'al line. Althoughthis arrangement is preferred at present, it will be apparent that asingle transmitting antenna may be used, with switched receivingantennas as illustrated by Figure 2, or both the transmitting andreceiving antenna systenis may loev switched to provide' overlappingIdirective patterns.

As amatter of practice, it is not possible to arrange the' switches 25and 21. tol change the oonnectionsof the transmitter I instantaneouslyfrom one' of the antennas to the other. The design-er confronted withthe choice of having both antennas connected to the transmitter duringthe switching period,` or having'both antennas disconnected from thetransmitter during the switching period. The former alternative is' tobe preferred', since tn'ozvariation of tho'impedanoe presented toftne'transmitter ty' this' method is' only 2:1.v A similar choice must be'madel with reference to the switch! 49. In this case", the movingContact' 1'1 must .be diiscdrlnected from both ofthe stationary contactsduring the switching period, rather than connectedl to both of them,since the capacitor would otherwise be' discharged durinfg eachoperation o'f the'switchpoducing serious transient disturbances'. Itshould be notedl that the length'sof the periods during which the switch49' is closed in each of its twol position-s' will aliect onily thesensitivity of they system, since the capacitor 41 will not be fullycharged within the time during which the contact Tl is connected toeither side. However, since the average charging current must equal theaverage discharge, the calibration is independent of the relativedurations of the connection periods. Thus, no serious requirement isplaced uponV the switch 49 with regard to matching the lengths of theperiods of alternate connection.

The relays 53 and 55 are adjusted to provide dead space in order toprevent spurious operation of the motor 59 as a result of noisevariation in tube characteristic, etc. A single relay provided withequivalent arrangement may be substituted for the relays 53 and 55.

Thus the invention has been described as a radio target seeking deviceincluding an improved comparator circuit in which the amplitudes ofalternately recurring sign-als are compared by charging a capacitor inresponse to one of said signals, and periodically connecting saidcapacitor in series with a voltage produced by the other of said signalsto a utilization circuit.

I claim as my invention:

l. In a radio locator system including radio transmitter4 means andradio receiver means whereby signals are radiated to and received from areflecting object, and means for cyclically changing the directiveresponse of said system from one to the other of two alternateoverlapping directive patterns, means for comparing the amplitude of theoutput of said receiver means during periods of operation with one ofsaid directive patterns with the amplitude of the output of saidreceiver means during periods of Voperation with the other of saiddirective patterns, said amplitude comparing means comprising arectifier connected to said receiver means, a capacitor with oneterminal connected to one side of the output circuit of said rectier andanother terminal connected to a double throw switch, one contact of saidswitch being connected to the other output terminal of said rectifierand the other contact of said switch being connected to a utilizationcircuit including D.C. polarity-responsive means.

2. The invention as set forth in claim 1 wherein said D.C.polarity-responsive means includes a. D.-C. amplifier and relay meansconnected to the output circuit of said amplier.

3. The invention as set forth in claim 1 wherein said double throwswitch is operated synchronously with said cyclical changing ofdirective response.

4. In a radio locator system including means for producing in a commonoutput circuit two trains of voltage pulses, interlocking in such mannerthat the pulses of one train alternate with those of the other, meansfor comparing the amplitudes of said pulses including a capacitor, meansfor connecting said capacitor across said common output circuit insynchronism with the pulses of one of said trains, a utilizationcircuit, and means for connecting said capacitor in series with said'common output circuit and said utilization circuit in synchronism withthe pulses of the other of said trains.

5. A voltage amplitude comparator system for radio target seekingdevices and the like, including means for cyclically and alternatelyproducing in a common output circuit two voltages to be compared, acapacitor, means for connecting said capacitor across said common outputcircuit substantially in synchronism with the occurrence of one of saidvoltages, a utilization circuit, and means for connecting said capacitorin series with said output circuit and said utilization circuitsubstantially in synchronism with the occurrence of the other of saidvoltages.

6. A radio locator system including means for directively transmittingand receiving signals in two overlapping directive lobes alternately,rectier means connected to said receiver, a capacitor, a utilizationcircuit, and means for alternately connecting said capacitor across theoutput circuit of said rectier means and in series with the outputcircuit of said rectier means and said utilization circuit.

WILLIAM R. MERCER.

REFERENCES- CITED The following references are of record in the le of'this patent:

UNITED STATES PATENTS Number Name Date 2,176,469 Moueix Oct. 17, 1939

